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September 8, 2020 – Critical fire weather concerns into Wednesday with gusty north to east wind and low humidity. Smoke and haze from wildfires will continue to impact air quality and temperatures. Red Flag Warning until noon PDT Wednesday, Wind Advisory until 6 PM PDT this evening.

Discussion

Gusty north to east winds continue to increase and spread southward early this morning as surface pressure gradients tighten behind a dry cold front dropping south through the Great Basin. The MFR-SAC and RNO-SAC gradient are both currently around 10.5 mbs. Northerly wind gusts of 30 to 45 mph have spread into the northern Sacramento Valley and surrounding foothill and mountain terrain.

Winds will spread south through the Central Valley early this morning, and gusts will increase in the Sacramento and Stockton areas by mid-morning as mixing erodes the nocturnal radiative inversion. Currently looks like the strongest winds will occur by midday as the north and east surface gradient maxes out (MFR- SAC and RNO-SAC both forecast to reach 12-13 mbs), and 925 mb winds increase to around 40 kts with 850 mb winds reaching 50 kts.

These winds combined with the recent very dry and record hot weather will result in critical fire weather conditions that will last into Wednesday.

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Surface gradients forecast to slacken a bit tonight allowing winds in the Central Valley to weaken, though breezy north winds will continue overnight along the western edge of the valley. Strong east gradient will continue to result in gusty winds over the foothills and west slopes of the northern Sierra, but likely not as strong as early this morning for most areas.

North and east winds decrease by Wednesday afternoon, but it will probably be until Thursday or Friday when onshore flow begins to spread higher RH inland. Temperatures will significantly lower today, but will rebound slightly the remainder of the week.

Extended Discussion (Saturday through Tuesday)

Upper ridge over the area shifts into the Great Basin Sunday as short wave trough in the EPAC approaches. Wave moves through early next week resulting in increased onshore flow and cooling. High temperatures are about 3-7 degrees above normal Saturday cooling to near or slightly below normal Monday into Tuesday.